Dynamic attribute creation and fetching

This pull request enables the possibilities for adding a file with dynamic attributes (instead of only filename previously). It is implemented via the insert_with_bson/2 function.

I've also created a simple function for the cursor, inorder to allow getting values of attributes dynamically.

I didn't build this for the sole purpose of pull requesting (hence, the strange early commit messages) ...
But I figured it's something someone else might want aswell.